Y-chromosomes and Father's Ancestral Line
Sex chromosomes - X and Y
The sex of a child is determined by the 23rd pair of human chromosomes, the "sex chromosomes", XX for a girl and XY for a boy. So, as the Y-chromosome can only be passed on father to son, a Y-chromosome test will show the father's ancestral line via grandfather, great-grandfather etc. Normally this will also help in finding male relatives with the same surname.
Consider a direct male ancestor with your surname a few generations back, who we will refer to as Mr A. Their Y-chromosome will be passed to their son, their son's son etc down to your father. So all your direct male ancestors with the same surename should have the same Y-chromosome as your father.
Recorded Lineage and Biological LineageSuppose that Mr As wife has a son, Mr A junior who is actually the biological son of a Mr B. This would mean that even though the male heirs from that moment on would have the surname A, they would all carry the Y-chromosome inherited from Mr B. Hence, your surname would only reflect the recorded lineage of your family, not the actual biological or Y-chromosome lineage of Mr B.
The further back you go in time the more likely this might have happened so you can't be entirely sure that you are related by surname to all others with the same surname. Indeed, your real ancestor's surname would be B (or C, D etc depending on how many times in the past this may have happened). Fortunately the story from your mothers line is more straightforward, see Mitochondrial DNA and Mother's Ancestral Line.
If you want to find more about the history of your surname do a Y-chromosome test but be prepared for a surprise. This is just another aspect of why DNA testing is so fascinating and can sometimes reveal long lost family secrets which you may or may not want to know!
DNA And Your Irish Roots
• DNA and Irish Roots
• DNA, Discovery and Functions
• Types of DNA tests
• DNA testing companies
• Y-chromosome and Father's Ancestral Line
• Mitochondrial DNA and Mother's Ancestral Line
• DNA and Ethnicity